Answer
For MDF edges, sand with 220, seal or prime, then scuff with 320 after it dries.
Why it happens
MDF edges absorb sealer and primer more than the face, and the fibers can raise during prep.
Recommended grit
Use 220 before sealing, then 320 after the sealer or primer dries.
Wet or dry
Dry.
Success check
The edge feels smooth and sealed before the topcoat.
What to do
- Sand the MDF edge with 220 grit.
- Remove dust completely.
- Apply sealer or primer to the edge.
- Scuff the dry sealed edge with 320.
Avoid: Do not paint raw fuzzy MDF edges without sealing.